#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;

int main()
{
    double a, b, c, d;
    cin >> a >> b >> c >> d;
    double t1 = a / b, t2 = 1 - c / d;
    double ans = 0;
    double t = 1;
    for(int i = 0; i < 1000000; i++) {
        ans += t * t1;
        t = t * (1 - t1) * t2;
    }
    printf("%.8lf", ans);
    return 0;
}